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178
RIPASSO della circonferenza
tramite il sito http://m.everythingmaths.co.za
 arc: an arc is a part of the circumference of a circle.
[Circumference si pronuncia sə’kʌmfərəns, con
accento dopo l’apostrofo ossia sulla 2a sillaba]
 chord: a straight line joining the ends of an arc.
 radius: a radius, r, is any straight line from the centre
of the circle to a point on the circumference.
 diameter: a diameter, ∅ , is a special chord that
passes through the centre of the circle.
[Pronuncia daiˈæmitə,
accento dopo l’apostrofo]
 segment: a segment is the part of the circle
that is cut off by a chord.
A chord divides a circle into two segments.
 tangent: a tangent is a line that makes contact with
a circle at one point on the circumference
(AB is a tangent to the circle at point P)
Quali sono i termini corrispondenti in Italiano?
The line drawn from the centre of a circle,
perpendicular to a chord,
bisects the chord.
Come si dimostra? Si tracciano …
The perpendicular bisector of a chord
passes through the centre of the circle.
Che parola italiana usiamo di norma al posto
dell’espressione inglese “perpendicular bisector”?
Come si dimostra l’enunciato?
179
RIPASSO della circonferenza
tramite il sito www.mathsisfun.com
Tangent Angle
A tangent is a line that just touches a circle at one point.
It always forms a right angle
with the circle's radius as shown here.
Ti ricordi la dimostrazione (per assurdo)?
Inscribed Angle:
an angle made from points sitting
on the circle's circumference
A and C are "end points"; B is the "apex point"
Inscribed Angle Theorems
An inscribed angle a°
is half of the central angle 2a°
(called the Angle at the Center Theorem)
Sapresti fare la dimostrazione
con riferimento alla figura qui a fianco?
Ti ricordi quali sono gli angoli alla circonferenza
“di seconda specie”?
Vale anche per essi il teorema?
And (keeping the endpoints fixed)
the angle a° is always the same,
no matter where it is on the circumference
(Angles Subtended by Same Arc Theorem)
Perché questo teorema è un corollario del precedente?
180
Angle in a Semicircle
An angle inscribed in a semicircle
is always a right angle
(the end points are either end of a circle's diameter,
the apex point can be anywhere on the circumference)
Ti ricordi come si dimostra?
Cyclic Quadrilateral
A "Cyclic" Quadrilateral
has every vertex on a circle's circumference
A Cyclic Quadrilateral's opposite angles add to 180°:
a° + c° = 180°; b° + d° = 180° [gradi = degrees]
Ti ricordi come si dimostra?
E qual è, invece, la proprietà caratteristica
dei quadrilateri CIRCOscritti?
Dimostrala, utilizzando la figura qui a destra →
V, W, X and Y →
are points on the
circumference.
Chords VX and WY
intersect
at the point Z.
^ XVW = 72°
and ^VXY = 38°.
What is the size
of ^VZW?
Le RISPOSTE
sono le soluzioni
delle seguenti equazioni:
( x + 12)2 = x2 + 4( 5x + 106) ↑
3 ( x + 1) + 64 = 5 ( x − 1) →
→
RS and RT are tangents
to the circle center O.
^SUT = 72°
What is the size
of ^SRT?